That machinist made a mistake (ok, more than one). You can make all these 'ghost guns' you want. Nothing illegal there (assuming you're not a felon or such). No serial number, nothing traceable. In fact it's easy (if you know how to run a mill & are handy). You can learn the basics of a mill in an hour, and the first one will take an hour or three to finish milling (staring with an 80% lower). Add another hour or two assembling your 'ghost' AR15. Yup, your first one will not be very good, call it a learning run. But I'll wager that machinist didn't have that learning curve.
The mistake is that you can't sell them. Never. Not one. You can't even give them away. You can't put them in your will. I'm not sure you can even legally let other people fire them. This issue is only the lower, the rest of the gun could be disassembled from the lower & sold (recover $550 out of $600?). I've seen 80% lower sell for as little as $50 i recent years, so you're not really losing that much other than a little time.
The lesson here is that gov't thugs are always trying to entrap people. They can't get the evidence to nail you on what they don't like (or it's simply not a crime), so they find another way. This is what started Ruby Ridge, undercover gov't agents spent months begging Randy Weaver to make them a sawed off shotgun. He had said "no" over and over and over and over. But he was broke and desperate, and finally the cash was too tempting. So they had him and they changed the court date without notifying him and that lead to the killing of his son and wife. The gov't was sued (but of course nobody went to jail) and paid Randy $3million as their valuation of his wife and son.
This machinist needs a good lawyer, and he needs to claim the she-man Janet Reno defense. If the federal gov't can let thousands of weapons go straight into the hands of the drug cartel with no legal consequences, why is this machinist due any worse? Rub his reputation in the mud a little, shake your finger at him, and let him go.
Oh, he made one more mistake. The machinist should have stated to the undercover agents that he was only making these guns as part of the Fast & Furious program, and record that conversation to present in court. If I was on the jury, I'd consider it!